Next and John Lewis, two of Britain's fastest-growing retailers, have warned of a slowdown on the high street amid fragile – albeit stable – consumer confidence.

Next, Britain's second largest clothing retailer after Marks & Spencer, admitted August and September had been disappointing and an "unusually quiet period". Its shares plunged to close down more than 7%. The chain made a better than expected start to the year, with revenues up 4.8% and profit 10.2% ahead of last year in the first half.

M&S chief executive Marc Bolland, meanwhile, said stock management problems that have hit womenswear sales for six months had been fixed. Addressing a Reuters retail summit, he dismissed talk of a takeover bid and said major investors backed his strategy of focusing on the UK.

Next's chief executive, Lord Wolfson, questioned whether the disappointing late summer was a one-off due to the Olympics or a sign of wider problems.

Analysts estimate that brand sales fell about 3% in the past five weeks because of the Olympics, Paralympics and holidays.

"Is it the general economy slowing down, or is it one-off effects?" asked Next chief executive Lord Wolfson asked. He suspected the "last six weeks are not indicative of a significant slowdown in the economy", but painted a picture of "subdued stability, with same-store sales moving backwards" – though he said new shop space and growth in online sales would make up for that.

He noted the gap between inflation and wage increases was narrowing, but thought this would take time to work through. "If the economy had a weather forecast, the outlook would be overcast – patchy rain for the foreseeable future. In the run-up to the credit crunch, individuals, businesses and government lived beyond their means. It will take some time to work our way back to affording the lifestyle to which we became accustomed."

Richard Hunter, head of equities at stockbroker Hargreaves Lansdown, said: "Next's outlook statement has proved to be its undoing. Yet the share price reaction masks another set of solid numbers."

As well as expanding its profitable international online business, Next will introduce same-day, evening, Sunday and next-day-to-store deliveries in Britain over the next six months.

It returned a further £112m to shareholders through share buybacks and increased the interim dividend by 12.7% to 31p a share.

John Lewis said like-for-like sales at its stores rose 8.5% in the past six weeks, down slightly from the stellar 9.2% seen in the first half of the year. Its grocery chain, Waitrose, was 4.5% ahead in recent weeks, compared with 2.2% growth in the first half. The partnership made a half-year profit before tax of £144.5m, up 60% from last year, having gained market share in home, fashion and electricals.

"Consumer demand remains fragile, but has stabilised and we continue to see opportunities to grow market share," said John Lewis's chairman, Charlie Mayfield. "Our rate of growth will remain positive but will be slower in the second half and, with further investment planned in that period to strengthen our business for the longer term, the rapid rate of profit increase is not expected to be carried through to the full year."

Managing director Andy Street added that he believed the mood of uncertainty had passed, for now. "Customers think they can see the economy and personal finances panning out. We are not expecting any shock to this."

John Lewis has, for the first time, trialled a "pop-up" shop in Exeter, ahead of the opening of a department store next month.

Home Retail Group, the third retailer to report on Thursday, said like-for-like sales at its Argos stores rose 1.4% in the quarter to 1 September, while DIY chain Homebase suffered a 3.7% decline because of wet weather.

Philip Dorgan, retail analyst at Panmure Gordon, said: "We believe that Argos has too many stores, too high a fixed cost base and is close to making losses."

Home Retail boss Terry Duddy defended Argos's "strong model". Having closed seven Argos stores in the quarter, the group plans to shut more over time while upgrading the Argos website. It has also started rolling out Habitat products such as paint and wallpaper in its Homebase stores.